Handle space-separated fingerprints for all relevant keywords

In version 3.2.0, we introduced spaces-separated fingerprint support for x-subscribe.

We should probably revisit this for the other relevant keywords, too. I just did x-fetch-key with such a fingerprint, and got, in return, ten times the message: Invalid input. Allowed are: URLs, OpenPGP-fingerprints, or email-addresses.. I'm not sure, and didn't checked, if this is an issue for other keywords too, but I guess so.

OTOH, as we spoke about "being implicit", I wonder if parsing fingerprints with spaces is maybe error prone?

Still, if we do, we should be consistent. Tagging this for 4.0 for now, and labeling as bug.